Medium-strength corrosion-resistant aluminum alloys



Patented June 17, 1952 UNITED STATES PATENT' OFFICE MEDIUM-STRENGTH CORROSION-RESIST- ANT ALUMINUM ALLOYS Thomas L. Fritzlen, Hinsdale, Ill., assignor to Reynolds Metals Company, Richmond, Va., a

corporation of Delaware No Drawing. Application October 22, 1948, Serial No. 56,057

Claims. ,(01. 75447) My alloy is as follows:

TABLE I Per cent Si .50- .90 Mg .8- 1.2 Mn .25- .75 A1 and impurities Balance TABLE II Mechanical properties on .064" sheet after solution heat treatment for 15 minutes at tempera.-

ture I Tensile Yield Per Cent Solution Temperature ZE??? Elongation sq. in. so. in. m 2

NorE:Above mechanical properties obtained 36 hours after solution heat treatment.

After solution heat treatment, my sheet may be quenched as in cold water, rolled level and flattened, stretched, side sheared and subjected to a precipitation heat treatment of about 18 hours at 320 F. The mechanica1 properties obtained after solution heat treatment and precipitation heat treatment also vary with the temperature employed for solution as shown in Table III.

TABLE III Mechanical properties of .064" sheet after solution and precipitation heat treatments Tensile Yield Per Cent Solution Temperature gaggi g, Elofiigzgt iou sq. in. sq. in.

No'rE.Solution heat treatment consisted of 15 minutes soak at temperature. Precipitation heat treatment was for 18 hours at 320 F My alloy sheets obtain their strength through the said solution heat treatment and precipitation heat treatment at elevated temperatures.

Formed aircraft parts may be made from the sheet, prior to heat solution and precipitation treatment, and these parts given solution heat treatment, say at 935 F., followed by controlled, relatively slow, quenching, as a guard against distortion and subsequent aging, say for 18 hours at 320 F., or for 8 hours or for 6-10 hours at 340-350 F. The solution heat treatment, quenching and aging may vary in details from the examples given.

Maximum tensile properties can be obtained upon precipitation heat treatment at any time after solution treatment and quenching. Also, no consistent effect upon corrosion resistance has been noticed when precipitation heat treating at various periods after solution heat treatment.

My product can be formed, hammered, rolled, or drawn, then solution and precipitation heat treated to develop maximum tensile properties in the T condition. Working of the sheet can also be carried out at any time after solution heat treatment followed by precipitation heat treatment to develop maximum properties.

My alloy possesses equally good corrosion resistance in all tempers, i. e., annealed, as rolled,

solution heat treated, and fully heat treated by both solution and precipitation treatments. The mechanical properties of fully heat treated sheet before and after corrosion are shown in Table IV.

TABLE IV Mechanical properties of .064" sheet prior to and after corrosion 1. Corrosion consisted of a 6-hour immersion in a solution 01': 57 grams of sodium chloride ml. of 30% hydrogen peroxide Balance of liter-distilled water.

Specimens were cleaned prior to corrosion by immersion for l min. at 95 degrees C. in:

50 ml. of concentrated nitric acid (70%) 50 ml. of hydrofluoric acid (48%) Balance of liter-distilled water.

After cleaning, specimens were rinsed in distilled water, immersed one minute in concentrated nitric acid, rinsed in distilled water and dried.

After corrosion, specimens were cleaned by immersion in 70% nitric acid, rinsed in distilled water and dried by air and dessication.

2. All of above specimens exhibited pitting type of attack only upon microscopic examination.

3. Corrosion carried out at constant temperature of 75 degrees F.

The corrosion test applied may be compared roughly to 2 to 4 years exposure to salt air, or ocean salt spray mist.

In my alloy, 1.00% magnesium is combined with approximately 0.6% silicon to form magnesium-silicide. This leaves an excess of about 0.10% silicon over and above that required to form magnesium-silicide; this silicon combines with the iron at a ratio of about 1 part of silicon to four parts of iron, and since manganese combines with iron with the same propensity as silicon, a constituent Al-Fe-Mn-Si is formed.

All of the silicon present in the R301 cladding alloy will be either a magnesium-silicide (MgzSi) or AlFeMn--Si constituent. This alloy is not susceptible to intergranular corrosion when solution heat treated and artificially aged, as borne out by numerous corrosion tests in salt spray and in the standard salt-hydrogen peroxide solution.

The manganese combines with the iron and silicon to form AlFe--MnSi constituent thereby imparting a superior resistance to pitting corrosion. It also increases the hardness and toughness as well as imparting a grain refining effect.

The iron content in my alloy runs between 0.40 percent and 0.60 percent (maximum). Since there is about 0.10 silicon to satisfy about 0.40 percent iron, and since it requires about 2 parts of manganese to combine with 1 part of iron, at least 0.25 percent manganese is required, and 0.50 percent is preferred, to take care of possible variations in magnesium, silicon and iron. The upper limit of 0.75 percent was selected in order not to increase fabricating difficulties of this alloy.

It was also found that manganese tended to increase the potential (electronegatively) of the alloy.

Summarizing, my alloy is not susceptible to intergranular corrosion when solution heat treated and artificially aged and possesses a corrosion resistance only slightly less than that of pure aluminum due to the proportion of magnesium to silicon and the presence of manganese.

Having described my invention, what I claim and desire to secure by Letters Patent is as follows:

1. An aluminum base alloy composed substantially entirely of Si, above .50 and below Mg, .8-1.2% and Mn, .25.75%; Fe, above .40 and below .60% and the balance aluminum wherein the magnesium is combined with the silicon, leaving an excess of at least 0.10 per cent excess silicon combined in the form of an A1-FeMn-Si constituent.

2. An aluminum base alloy composed substantially entirely of Si, above .50 and below 90%; Mg, .8-1.2% and Mn, .25-.75%; Fe, above .40 and below .60% and the balance aluminum, any impurity content that may be present containing not over Cu, .05%, Cr, 25%, other impurities .05% maximum each and wherein the magnesium is combined with the silicon, leaving an excess of at least 0.10 per cent excess silicon combined in the form of an Al-FeMn-Si constituent.

3. The alloy of claim 1 in the heat treated state produced by heating the said alloy to about 920-980 F. for about 15-20 minutes, quenched in cold water, and then subjected to a precipitation heat treatment of about 18 hours at about 320 F.

4. The alloy of claim 2 in the heat treated state produced by heating the said alloy to about 920-980 F. for about 15-20 minutes, quenched in cold water and then subjected to a precipitation heat treatment of about 6-10 hours at 340- 350 F.

THOMAS L. FRI'IZLEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Engineering Alloys, published by the American Society for Metals, 1936, page 340".

Korrosionstabellen Metallischer Werkstoffe," by Ritter, lithoprinted by Edwards Bros, Inc., Ann Arbor, Mich., 1945, page 4.

Light Metals in Structural Engineering, by Dudley, published by Temple Press Ltd., London, pages 200 and 201. 

1. AN ALUMINUM BASE ALLOY COMPOSED SUBSTANTIALLY ENTIRELY OF SI. ABOVE .50 AND BELOW .90%; MG, .8-1.2% AND MN, .25-75%; FE, ABOVE .40 AND BELOW .60% AND THE BALANCE ALUMINUM WHEREIN THE MAGNESIUM IS COMBINED WITH THE SILICON, LEAVING AN EXCESS OF AT LEAST 0.10 PER CENT EXCESS SILICON COMBINED IN THE FORM OF AN AL-FE-MN-SI CONSTITUENT. 